As an example of a mobile communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) will be briefly described.
An LTE system is a mobile communication system evolved from a Universal Mobile Telecommunications System (UMTS) system and the standard thereof is established in the 3GPP. The schematic structure of the LTE system is shown in FIG. 1.
FIG. 1 is a diagram showing a network architecture of an LTE system which is an example of a mobile communication system.
The structure of the LTE system may be roughly divided into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC).
The E-UTRAN includes at least one eNB (Evolved Node B or base station). An interface between a UE and an eNB is referred to as a Uu interface and an interface between an eNB and another eNB is referred to as a X2 interface.
The EPC includes a mobility management entity (MME) for performing a control plane function and a serving gateway (S-GW) for performing a user plane function. An interface between an eNB and an MME is referred to as an S1-MME interface and an interface between an eNB and an S-GW is referred to as an S1-U interface. These interfaces may be collectively referred to as an S1 interface.
In a radio Uu interface, a radio interface protocol is defined. The radio interface protocol is horizontally divided into a physical layer, a data link layer and a network layer. The radio interface protocol is vertically divided into a user plane (U-plane) for transmitting user data and a control plane (C-plane) for signaling a control signal.
The radio interface protocol may be divided into L1 (first layer) including a physical layer (PHY), L2 (second layer) including a MAC/RLC/PDCP layer, and L3 (third layer) including a RRC layer based on the three lower layers of an open system interconnection (OSI) standard model which is well-known in the art of communication systems, as shown in FIGS. 2 and 3. A pair of radio interface protocols exist in a UE and an E-UTRAN to perform a data transmission function of a Uu interface.